Method of increasing the scaling resistance of metallic objects



METHOD OF INCREASING THE SCALING RESISTANCE F METALLIC OBJECTS Herbert Schulze and Hans Schriewer, Altena, Germany,

assignors to Vereinigte Deutsche Metallwerke Aktiengeselischaft, Frankfurt am Main, Germany No Drawing. Application May 18, 1953 Serial No. 355,869

Claims priority, application Germany May 23, 1952 11 Claims. (Cl. 117-71) The present invention relates to a process for imparting high scaling resistance to metallic objects especially those of steel of all kinds.

Many metal objects, especially those of iron, steel, copper or alloys thereof are frequently exposed to high temperatures as well as to the influence of corrosive gases, particularly gases which have a strong oxidizing action. The life of such metal objects is consequently relatively short and this necessitates undesirable frequent replacement of the scale encrusted parts, as, for example, in the case of grate bars, boiler plates or other parts of furnaces. Furthermore, other oven parts and also hardening and smelting crucibles, troughs and other vessels for salt baths; and in addition casting molds and other casting apparatus; annealing boxes, carburizing boxes, parts of smelting and roasting furnaces; superheaters; protective tubes or in general tubes and plates and many other articles used in industrial installations are often exposed to conditions giving rise to scaling.

It has already been proposed to reduce the scaling resistance of objects of structural steel or boiler plates by applying a protective coating of a chromium and nickel containing iron alloy and then spraying" on a coating of aluminum containing paint. Investigations, however, have shown that such protective coatings would not hinder extremely intense scaling of the objects when they were subjected to temperatures over 1000' C. after 235 hours.

It was unexpectedly found according to the invention, that excellent scaling resistance can be achieved against the corrosive influence of hot gases at temperatures over 800 C. and up to about 1100' C. if the metal objects are firstspray coated with chromium-nickel-iron or chromium-nickel alloys and such coatings are then provided with a coating of chromium oxide (C30,), lithium com pounds, especially lithium carbonate, lithium oxide and lithium hydroxide, or aluminum oxide.

The latter coating materials can be applied in a number of ways. -A simple procedure is to mix such materials in powder form with a lacquer or paint, preferably a bituminous base paint and then applying such lacquer or paint upon the sprayed metal coating.

It is also possible to suspend the lithium compounds, aluminum oxide and magnesium oxide either singly or in admixture in water and applying such suspension to the sprayed metal coating. Preferably, when the abovementioned materials are mixed the quantity of lithium compound, preferably lithium carbonate, amounts to to 20% and the remainder is aluminum oxide and magnesium oxide.

Mixtures of lithium carbonate with the aluminum oxide and/or magnesium oxide can also be applied in admixture with a lacquer. When suspensions in water are employed, care should be taken in preparing such suspensions so that a good coating can be obtained there- I with by painting it with a brush. The preferred ratio of um compound, particularly, lithium carbonate, to the um oxide and/or magnesium oxide is between Patented Feb. 11, 1958 Of the chromium and nickel containing alloys suitable for the first coating applied to the metal objects to be protected, chromiumnickel-iron alloys containing 18 to 30% of nickel, about 18 to 25% of chromium, up to 3% of silicon, and the remainder iron; or 60 to 65% nickel, 12 to 20% of chromium, up to 3% of silicon, and

the remainder iron; or chromium-nickel alloys containing to of nickel, 15 to 20% of chromium, and up to 2% of silicon, have given best results. The alloys mentioned can in addition contain up to 10%, of tungsten and/or molybdenum. Moreover, their resistance to heat may be improved in known manner by additions of alkaline earths, rare earths, thorium, zirconium, uranium, either singly or several together, while of each of the elements mentioned contents of from 0.02 to 0.5% may be present. The thickness of the sprayed metal coating to be applied is about 0.25 to 0.40 mm.

The following examples will serve to illustrate the process according to the invention.

Example 1 A coating of an alloy composed of 20% chromium, 30% nickel and the remainder essentially iron was sprayed on the articles to be protected and therafter an admixture of finely ground lithium carbonate powder with bituminous paint, was painted over the sprayed on metal coating. The quantity of lithium carbonate mixed with the bituminous paint was such that the mixture could easily be applied with a brush.

Example 2 A coating of an alloy composed of 20% chromium, 30% nickel and the remainder essentially iron was sprayed on the articles to be protected and thereafter a suspension of 20% lithium carbonate and 80% A1 0, in water was painted over the sprayed on metal coating.

Example 3 temperatures above 800' C. in the presence of corrosive gases of a metal article coated witha scaling resistant chromlum-and-nickel containing alloy which comprises applying to said coated metal article a coating consisting essentiallyof a material selected from the group consisting of chromium oxide, aluminum oxide, lithium oxide, lithium hydroxide and lithium carbonate.

2. A method according to claim 1 in which the thickness of the scaling resistant chromium-and-nickel containing alloy on the metal article is 0.25 to 0.40 mm.

3. A method according to claim 1 inwhich said coating on the chromium-and-nickel containing alloy coated article is applied in the form of-a bituminous paint.

4. A method according to claim 1 in which said coating applied to chromium-and-nickel containing alloy coated article contains lithium carbonate.

5. A method of improving the scaling resistance at temperatures above 800' C. in the presence of corrosive gases of a metal article coated with a scaling resistant chromium-and-nickel containing alloy which comprises applying to said coated metal article a coating in the form of an aqueous suspension of finely divided lithium carbomb.

6. A method of improving the scaling resistance at temperatures above 800 C. in the presence of corrosive gases of a metal article coated with a scaling resistant chromium-andnickel containing alloy which comprises applying to said coated metal article a coating in the form of an aqueous suspension of finely divided aluminum-oxide.

7; A method of improving the scaling resistance at temperatures above 800' C. in the presence of corrosive gases of a metal article coated with a scaling resistant chromium-and-nickelcontaining alloy which comprises applying to said coated metal article a coating in the form of an aqueous suspension of finely divided lithium carbonate and aluminum oxide.

8. A, process in accordance with claim 7 in which said lithium carbonate and aluminum oxide are present'in a proportion of from 1: 10 to 1': 1.

9. A method of improving the scaling resistance at temperatures above 800 C. in the presence of corrosive gases of a metal article coated with a scaling resistant chromium-and-nickel containing alloy which comprises applying to said coated metal article a coatingin the form of an aqueous suspension of finely divided lithium carbonate and magnesium oxide.

10. Aprocess in accordance with claim 9 in which said lithium carbonate and magnesium oxide are present a proportion'of from 1:10 to.1:1.

References Cited in the file of this patent. UNITED STATES PATENTS 1,8965041: Ruben Ian. 31, 1933 2,300,400 Axline June 26, 1940 2,457,515 Aker Dec. 28, 1948 2,697,670 Gaudenzi Dec. 21, 1954 2,730,458 Schultz Jan. 10, 1956 FOREIGN PATENTS 505,634 Belgium Sept. 29, 1951 OTHER REFERENCES T-upholme as a Protective Coating, News Edition orindustrial and,;,E g neering Chemistry, May 20, 1936; vol. 14, No; 10,195; 

1. A METHOD OF IMPROVING THE SCALING RESISTANCE AT TEMPERATURES ABOVE 800*C. IN THE PRESENCE OF CORROSIVE GASES OF A METAL ARTICLE COATED WITH A SCALING RESISTANT CHROMIUM-AND-NICKEL CONTAINING ALLOY WHICH COMPRISES APPLYING TO SAID COATED METAL ARTICLE A COATING CONSISTING ESSENTIALLY OF A MATERIAL SELECTED FROM THE GROUP CONSISTING OF CHROMIUM OXIDE, ALUMINUM OXIDE, LITHIUM OXIDE, LITHIUM HYDROXIDE AND LITHIUM CARBONATE. 